Heat-sensitive recording materials that depend for their operation on a color development reaction between a colorless or pale colored electron donating dye precursor and an electron accepting compound are generally referred to as "two-component" color forming heat-sensitive recording materials, and examples of such recording materials are disclosed in Japanese Patent Publication Nos. 14039/70 and 4160/68 and Japanese Patent Application (OPI) No. 27253/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
To make the "two-component" color forming heat-sensitive recording material, a dispersion of fine particles of a colorless or pale colored electron donating dye precursor and an electron accepting compound is mixed with the particles of a binder, etc., to form a coating solution which is applied to a support in such a manner that the two thermal reactive compounds remain separated from each other by the particles of binder, etc. The resulting coating forms an image by making use of a color forming reaction that is initiated by contact between the two compounds when one or both of them are melted with heat.
The "two-component" color forming heat-sensitive recording material has great practical value since it has the following advantages: (1) it depends on the primary color forming reaction and requires no subsequent development step; (2) the texture of the material is similar to that of ordinary paper; (3) it is easy to handle; (4) it provides high color density; and (5) heat-sensitive recording materials providing various colors can be easily prepared. Because of these advantages, the use of "two-component" color forming heat-sensitive recording materials is rapidly increasing in facsimiles, recording instruments and printers. As the use of facsimile equipment is rapidly expanding today, efforts are being made to achieve higher recording speeds, and this requires a corresponding improvement in the heat response of heat-sensitive recording materials intended for use with facsimiles, i.e., more efficient color formation in response to short heat pulses, or low thermal energies.
In order to make the most of the capability of modern heat-sensitive recording systems to operate at high speed, studies on color densities and sensitivities have been actively conducted. Two of the methods so far described to attain this object include using an oxybenzoic acid ester as an electron accepting compound as described in Japanese Patent Application (OPI) No. 144193/81, and using a salicylic acid ester as described in Japanese Patent Application (OPI) No. 201693/82. However, the combinations of electron donating colorless dyes (also referred to as a color former) and electron accepting compounds (also referred to as a color developer) that satisfy the requirement of high color density are generally ineffective in meeting the requirements of no fog and adequate resistance of colored paper to moisture and heat. Hence, the provision of an additional means for preventing the occurrence of fog or color fading has been desired.
As already mentioned, the "two-component" color forming heat-sensitive recording materials depend on the primary color forming reaction for their operation, but because of this very fact, the reaction between an electron donating dye precursor and an electron accepting compound takes place not only when heat is applied, but also when these materials are exposed to solvents. This results from the fact that all of the existing "two-component" color forming heat-sensitive recording materials are made of organics and their solubility in certain solvents is so great as to easily cause undesired reactions in such solvents. As a matter of fact, if the heat-sensitive recording materials are brought into contact with writing devices containing such solvents, such as pens using aqueous inks, pens using oily inks, and fluorescent pens, or diazo developers, or adhesives or glues, the white background of the heat-sensitive recording sheet develops color, or the printed characters fade or completely disappear, thus greatly reducing the commercial value of such recording sheets. Conventionally, this problem is avoided primarily by providing a solvent resistant protective layer on the heat-sensitive color forming layer, but this adds to the number of manufacturing steps, and hence, the cost.
Another problem with the heat-sensitive recording materials, especially heat-sensitive recording paper, is that the color of the image once formed will fade or disappear entirely because of the adverse effects of such external conditions as moisture and heat. This is indeed a serious problem for heat-sensitive recording materials, and various proposals have been made with a view to solving this problem. Japanese Patent Publication No. 43386/76 describes the addition of a phenolic derivative such as 4,4'-thiobis(6-tert-butyl-3-methylphenol); Japanese Patent Application (OPI) No. 17347/78 describes the addition of a modified, say, rosin modified, water-insoluble phenolic resin; and Japanese Patent Application (OPI) No. 72996/81 proposes the addition of a terephthalic acid ester such as dimethyl terephthalate. These methods are to some extent effective in preventing the fading of color images but, at the same time, they cause "fog", or unwanted color formation during the manufacture or storage of heat-sensitive recording material. This fog problem is highly sensitive to moisture and heat, so the commercial value of heat-sensitive recording materials treated by any one of the methods described above will be appreciably decreased under hot or humid conditions.